The sydnone is a mesoionic 1,3-dipole. Its two cycloaddition termini add across the alkyne to give a bicyclic adduct. Subsequent retrocycloaddition expels the ring oxygen and carbonyl carbon together as carbon dioxide, leaving an aromatic pyrazole.
The product is 1--4,5-di--pyrazole, plus CO. The sydnone nitrogen bearing becomes pyrazole N1; the adjacent nitrogen becomes N2; the sydnone CH becomes C3; and the two alkyne carbons become C4 and C5. Since both alkyne substituents are the same, reversing its orientation gives the same constitutional product.
Formation of the aromatic pyrazole and release of CO favor the fragmentation. The net transformation can be understood entirely as a cycloaddition–cycloreversion sequence; no biological application is needed.
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